1. Technical Field
The present invention relates to an antenna and an electronic apparatus including the antenna, and particularly to a mobile or portable electronic apparatus.
2. Related Art
For example, in a satellite mobile phone or a navigation apparatus using GPS (Global Positioning System), wireless communication using a circularly polarized wave is performed. Although the circularly polarized wave can be received by a linearly polarized wave antenna, since the gain is halved, it is preferable to use a circularly polarized wave antenna. Besides, in an apparatus, such as a GPS equipment, which is mounted on a person or an animal and is used, it is difficult to always keep the maximum sensibility direction of the antenna in the direction toward the satellite. Thus, an antenna is desirable which is small and has a wide directivity, particularly a wide directivity while a specified circularly polarized wave characteristic is ensured. As an antenna to generate a circularly polarized wave, for example, there is a patch antenna mounted with a perturbation element of JP-A-2008-54080 (Patent Literature 1), a spiral antenna of JP-A-10-075114 (Patent Literature 2), or a curl antenna of a combination of a linear element and a curl element (JP-B-8-17289 (Patent Literature 3), H. Nakano et al “Axial ratio of a curl antenna” IEE Proc. Microw Antennas Propag., Vol. 144, No. 6, December 1997 (Non Patent Literature 1), H. Nakano et al “A Curl Antenna” IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 41, NO. 11, November 1, (Non Patent Literature 2)).
The patch antenna has defects that as the operation frequency becomes high, the perturbation element becomes small, and the manufacture becomes difficult, and further, the frequency band in which the required circularly polarized wave can be generated is very narrow. Besides, the directivity is single in the patch surface direction, and the beam characteristic becomes relatively narrow. When the ground electrode is made small, the so-called figure-eight directivity is obtained in which the beam is emitted also in the direction opposite to the patch surface (direction toward the ground electrode). However, the rotation direction of the circularly polarized wave to be transmitted and received is reversed between the patch surface direction and the ground electrode direction. Thus, in the patch antenna, it is difficult to obtain the wide directional pattern of the circularly polarized wave. Further, since the patch antenna uses a dielectric body, the loss of the dielectric body cannot be avoided. Even if the dielectric body with the lowest loss is used, the obtained radiation efficiency is about 30%.
Since the spiral antenna or the curl antenna is a travelling-wave type antenna, the antenna is generally large, and although the band is wide, the directivity is sharp, and application to a portable equipment is difficult.
In general, an antenna suitable for a portable equipment using a circularly polarized wave, such as GPS, is required to have the following performances.
1. The directivity is wide, and the circularly polarized wave having a low axial ratio over a wide spatial range can be transmitted and received.
2. The band is wide, and an extremely high component size accuracy is not required.
3. The size is small and the weight is light in order to enable mounting on a portable equipment.
4. The efficiency is high and the sensitivity is high.
Hitherto, there is no antenna satisfying all these requests.